This invention relates to a radial-ply tire with a block pattern tread.
For a radial-ply tire, a variety of tread patterns has been proposed. The block pattern in which a plurality of longitudinal grooves running in a circumferential direction of the tire and a plurality of transverse grooves intersecting the longitudinal grooves form a number of islands or blocks is found typically in a mud-and-snow tire and the tire with such a tread pattern exhibits its potential to the fullest extent on ice or snow and muddy roads.
In the radial-ply tire with such a block pattern tread, it has been proposed to lower bending stiffness of the block by providing it with a plurality of sipes. Reducing the stiffness of the block in this manner enables the block to softly contact a road surface so that further improvements can be accomplished in ice and wet performances.
However, when the sipes are linear, block segments isolated by these linear sipes are able to move independently so that lateral stiffness of the block is markedly compromised. Hence, the distortion of the blocks is increased so much that a vehicle with the tire totters at a high speed.
To overcome this disadvantage, it is effective to form each sipe in a wavy pattern. When a plurality of wavy sipes is formed in the block, each block segment as isolated by such sipes certainly comes into contact with an adjacent segment of the same block irrespective of a direction of distortion, so that the distortion of the respective block segments is delimited as compared with the case of linear sipes, resulting in an improved stiffness of the block as a whole. As a consequence, not only ice and wet performances of the tire are improved but a satisfactory high-speed running performance can be obtained.
When wavy sipes of the same pattern are formed at equal intervals in the respective blocks of a radial-ply tire, it is impossible to insure a uniform contact of each block with a road surface. Thus, bending stiffness of any block segment that lies in a peripheral region of the block is then lower than that of a segment of the same size which lies in a central region of the block. This is because, whereas the central block segment has an adjacent block segment on either side, the block segment in the peripheral region has no adjacent segment on one side.
To insure uniformity of rigidity within each block with the foregoing taken into consideration, it is considered effective to increase density of sipes in the central region over that in the peripheral region. In such an arrangement, the block is more finely divided in the central region of high sipe density than in the peripheral region of low sipe density so that bending stiffness of the central segment only is relatively reduced to keep a balance.
However, when the sipe density in the central region is thus increased over that in the peripheral region, side edges of the block are worn (toe-and-heel wear) earlier than the center of the block so that an uneven wear within the block reduces tire life. In addition, tire vibrations are amplified.
The object of this invention, which has been accomplished against the above background, is to provide a radial-ply tire whose tread has a pattern of blocks as divided by a plurality of longitudinal grooves running in the circumferential direction of the tire and a plurality of transverse grooves intersecting the longitudinal grooves and each of the blocks being provided with a plurality of zigzag or wavy sipes, which tire has been improved in that distribution of stiffness in the respective blocks is rendered uniform while variation in wear within the respective blocks and tire noise are successfully reduced.